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//===--- ReadOnlyGlobalVariables.swift ------------------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2023 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
import SIL
/// Marks global `var` variables as `let` if they are never written.
///
/// Note that this pass relies on the initialize-static-globals pass which converts lazily
/// initialized globals to statically initialized globals.
/// This pass does not mark lazily initialized globals as `let`, because such globals _are_
/// written: in their initializers.
///
let readOnlyGlobalVariablesPass = ModulePass(name: "read-only-global-variables") {
(moduleContext: ModulePassContext) in
var writtenGlobals = Set<GlobalVariable>()
for f in moduleContext.functions {
for inst in f.instructions {
if let gAddr = inst as? GlobalAddrInst {
if findWrites(toAddress: gAddr) {
writtenGlobals.insert(gAddr.global)
}
}
}
}
for g in moduleContext.globalVariables {
if !g.isAvailableExternally,
!g.isPossiblyUsedExternally,
!g.isLet,
!writtenGlobals.contains(g) {
g.setIsLet(to: true, moduleContext)
}
}
}
private func findWrites(toAddress: Value) -> Bool {
var walker = FindWrites()
return walker.walkDownUses(ofAddress: toAddress, path: UnusedWalkingPath()) == .abortWalk
}
private struct FindWrites : AddressDefUseWalker {
mutating func leafUse(address: Operand, path: UnusedWalkingPath) -> WalkResult {
switch address.instruction {
case is LoadInst, is LoadBorrowInst:
return .continueWalk
case let ca as CopyAddrInst:
return address == ca.sourceOperand ? .continueWalk : .abortWalk
case let apply as FullApplySite:
if let calleeArgIdx = apply.calleeArgumentIndex(of: address) {
let convention = apply.calleeArgumentConventions[calleeArgIdx]
if convention.isIndirectIn {
return .continueWalk
}
}
return .abortWalk
default:
return .abortWalk
}
}
}
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